Why do the buildings Collapse
Buildings, like all structures, are designed to support certain loads
without deforming excessively. The loads are the weights of people and objects,
the weight of rain and snow and the pressure of wind--called live loads--and
the dead load of the building itself. With buildings of a few floors,
strength generally accompanies sufficient rigidity, and the design is mainly
that of a roof that will keep the weather out while spanning large open spaces.
With tall buildings of many floors, the roof is a minor matter, and the support
of the weight of the building itself is the main consideration. Like long
bridges, tall buildings are subject to catastrophic collapse.
The causes of building collapse can be classified under general headings to
facilitate analysis. These headings are:
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Bad Design
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Faulty Construction
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Foundation Failure
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Extraordinary Loads
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Unexpected Failure Modes
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Combination of Causes
Bad design does not mean only errors of computation, but a failure to take
into account the loads the structure will be called upon to carry, erroneous
theories, reliance on inaccurate data, ignorance of the effects of repeated or
impulsive stresses, and improper choice of materials or misunderstanding of
their properties.
Faulty construction has been the most important cause of structural failure.
Some of the most general construction faults are
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Includes
the use of salty sand to make concrete
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The substitution of inferior steel
for that specified
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In case of steel structures bad
riveting or even improper tightening torque of nuts
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Excessive use of the drift pin to
make holes line up.
Even an excellently designed and constructed structure will not stand on a bad
Foundation. Although the structure will carry
its loads, the earth beneath it may not. The Leaning
Tower of Pisa is a famous example of bad foundations, but
there are many others. The old armory in St. Paul,
Minnesota, sank 20 feet or more into soft clay, but did not collapse.
The displacements due to bad foundations may alter the stress distribution
significantly. This was such a problem with railway bridges in America that
statically determinate trusses were greatly preferred, since they were not
subject to this danger.
Extraordinary loads are often natural, such as repeated heavy snowfalls, or
the shaking of an earthquake, or the winds of a hurricane. A building that is
intended to stand for some years should be able to meet these challenges. A
flimsy flexible structure may avoid destruction in an earthquake, while a solid
masonry building would be destroyed. Earthquakes may cause foundation problems
when moist filled land liquefies.
Unexpected failure modes are the most complex of the reasons for collapse,
and we have recently had a good example. Any new type of structure is subject to
unexpected failure, until its properties are well understood. Suspension bridges
seemed the answer to bridging large gaps. Everything was supported by a strong
cable in tension, a reliable and understood member. However, sad experience
showed that the bridge deck was capable of galloping and twisting without
restraint from the supporting cables. Ellet's bridge
at Wheeling collapsed in the 1840's, and the Tacoma
Narrows bridge in the 1940's, from this cause.
The conservative, strong statically determinate trusses were designed with
pin-connected eyebars to be as strong and safe as possible. Sad experience
brought the realization of stress concentration at the holes pierced in the
eyebars. It has been recognized
that tension members have no surprises. They fail by pulling apart when the
tension in them becomes too high. If you know the tension, then proportioning a
member is easy. A compression member, a column, is different. If it is short and
squat, it bears its load until it crushes. But if you try to support a load with
a 12-foot column that will just support the load with a 1-foot column, you are
in for a surprise. The column Buckles,
and the load crashes to earth.
Suppose you have a beam supported at the ends, with a load in the center.
You know the beam will bend, and if the load is too great, it may break apart at
the bottom, or crush at the top, under the load. This you expect. However, the
beam may fail by splitting into two beams longitudinally, or by the top of the
beam deflecting to one side or the other, also called buckling. In fact a beam
will usually fail by shearing or buckling before breaking.
A HOLLOW TUBE
makes a very efficient
column or beam. It is the material on the surface that most resists buckling and
bending. A column that is modified from a compact cross-section, like a
cylinder, to an extended cross-section, like a pipe, can still support the same
load per unit area, but with much greater resistance to buckling. As a beam, one
side is in compression and the other in tension, while the pipe cannot buckle to
one side or the other. When you do bend a pipe, notice that it crushes inward
reducing the cross-section to a line, which bends easily. Tubes need to be
supported against buckling. Such a tube has a very high ratio of strength to
weight, and hence strength to cost.
Tall buildings have generally been made with a rigid steel skeleton,
sheathed in the lightest materials to keep out the weather. Alternatively,
reinforced concrete, where the compression-resisting and protecting concrete
surrounds the tough, tension-resisting steel, integrated into a single body, has
been used. Such structures have never failed (when properly built on good
foundations), and stoutly resist demolition. When the lower supports of a steel
skeleton are destroyed, the weight of the building seems to crush the lower
parts and the upper parts descend slowly into the pile of debris. Monolithic
reinforced-concrete buildings are difficult to demolish in any fashion.
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